Regulation Text
(a) Each person operating an aircraft shall maintain the cruising altitude or flight level of that aircraft, as the case may be, by reference to an altimeter that is set, when operating—
(1) Below 18,000 feet MSL, to—
(i) The current reported altimeter setting of a station along the route and within 100 nautical miles of the aircraft;
(ii) If there is no station within the area prescribed in paragraph (a)(1)(i) of this section, the current reported altimeter setting of an appropriate available station; or
(iii) In the case of an aircraft not equipped with a radio, the elevation of the departure airport or an appropriate altimeter setting available before departure; or
(2) At or above 18,000 feet MSL, to 29.92″ Hg.
(b) The lowest usable flight level is determined by the atmospheric pressure in the area of operation as shown in the following table:
Current altimeter setting Lowest usable flight level 29.92 (or higher) 180 29.91 through 29.42 185 29.41 through 28.92 190 28.91 through 28.42 195 28.41 through 27.92 200 27.91 through 27.42 205 27.41 through 26.92 210(c) To convert minimum altitude prescribed under §§ 91.119 and 91.177 to the minimum flight level, the pilot shall take the flight level equivalent of the minimum altitude in feet and add the appropriate number of feet specified below, according to the current reported altimeter setting:
Current altimeter setting Adjustment factor 29.92 (or higher) None 29.91 through 29.42 500 29.41 through 28.92 1,000 28.91 through 28.42 1,500 28.41 through 27.92 2,000 27.91 through 27.42 2,500 27.41 through 26.92 3,000Research Notes
Section 91.121 governs altimeter settings — the rule that determines what your altimeter is telling you. Get this wrong and you're potentially thousands of feet off your indicated altitude.
Paragraph (a)(1) — Below 18,000 MSL: The current reported altimeter setting of a station along the route within 100 nautical miles of the aircraft. If no station is within 100 NM, the current reported altimeter setting of the appropriate station. This means you're flying with local altimeter settings throughout the lower atmosphere — you update your Kollsman window as you fly through different reporting stations.
Paragraph (a)(2) — At or above 18,000 MSL: 29.92 inches of mercury (or 1013.2 hPa). At and above FL180, every aircraft sets 29.92 — this is what creates Flight Levels. Above 18,000 feet, altitude is measured by indicated altitude on a 29.92 setting (FL180, FL190, FL200, etc.), not true altitude.
Paragraph (b) — IFR clearance climb-throughs: When climbing through the 18,000-foot transition altitude, the pilot resets to 29.92 at 18,000. When descending through, the pilot resets from 29.92 to the local altimeter setting at 18,000.
Paragraph (c) — When unable to obtain a reported altimeter setting: The PIC may use a setting that is appropriate for the area of operations, or an elevation if known (e.g., if at a known field elevation, set the altimeter to read that elevation). This is uncommon in routine operations but addresses isolated areas, ferry flights, and similar scenarios.
Why this matters operationally: Altimeter pressure changes 1 inch of mercury per 1,000 feet of altitude approximately. A 1-inch error in altimeter setting equals approximately 1,000 feet of altitude error in the indicated reading. The expression 'low to high, watch out below' captures the directional effect — flying from a high-pressure area into a low-pressure area without resetting means your altimeter over-reads (you're lower than indicated). 'High to low, look out below' is the same idea.
Cold weather corrections: Altimeters indicate true altitude only at standard temperature. In cold weather, true altitude can be SIGNIFICANTLY lower than indicated altitude. The AIM and the IFR Procedures Handbook include cold-temperature altitude correction tables; § 91.121 itself doesn't mandate cold-weather correction, but instrument procedures (e.g., LDA approaches into mountain airports in winter) explicitly require it.
Reference: AIM 7-2 on altimeter setting procedures including cold-weather corrections; IPH Chapter 4 on altimetry.
Amendment History
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